@AndyYan good point – but fastboot boot doesn't need a partition, just an image to load (and boot, one-time). Doesn't matter how the image file is named – could be xmastree.img though no Android device has an XMas-Tree built-in #D Which probably is why the response is "downloading 'boot.img'" (aka "I download something I hope I can boot from").
– Izzy♦Oct 20 '17 at 14:59